Process for producing composite substrate material

ABSTRACT

A functional film, which has photo-catalytic effects, is overlaid on a surface of each of at least two substrate materials. The functional film, which has been overlaid on the surface of each of the substrate materials, is exposed to light having a wavelength falling within an absorption wavelength range of the functional film. The substrate materials are then bonded to each other with the functional films, which have been exposed to the light, intervening between the substrate materials. The functional film may be a film containing TiO 2 , and the light irradiated to the functional film may be ultraviolet light. In cases where a film other than the functional film is also overlaid on the surface of each, of the substrate materials, the functional film is formed as a top layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a composite substrate material comprising atleast two substrates which are bonded to each other. This invention alsorelates to a process for producing the composite substrate material.

2. Description of the Related Art

Composite materials have heretofore been formed by bonding opticalcrystals, such as laser crystals, optical wavelength convertingcrystals, and quartz glass mirrors, to each other or by bonding opticalsubstrates to each other. As techniques for the bonding, techniques,wherein the optical crystals or the optical substrates are adhered toeach other by optical adhesive agents, or techniques, wherein theoptical crystals or the optical substrates are fusion bonded under heat,have heretofore been used widely.

However, the techniques, wherein the optical crystals or the opticalsubstrates are adhered to each other by optical adhesive agents, havethe problems in that optical scattering and reflection loss are causedto occur and long-term reliability of the adhered areas is low.Particularly, as for optical members to be located within laserresonators, the problems described above arise markedly.

The techniques, wherein the optical crystals or the optical substratesare fusion bonded under heat, have the problems in that the techniquesare applicable only to limited combinations of materials and can beutilized only in limited applications.

Therefore, recently, techniques for bonding different kinds of materialsto each other, which techniques are referred to as wafer bondingtechniques, have attracted particular attention. With the wafer bondingtechniques, as described in, for example, Japanese Unexamined PatentPublication No. 6(1994)-90061, wafers of single crystals or polycrystalsare subjected to mirror finish, the mirror surfaces thus obtained arewashed to remove dust and organic substances and are set in ahydrophilic state, the mirror surfaces are then brought into contactwith each other in a clean atmosphere, and the wafers are heated in thisstate.

With the wafer bonding techniques described above, a composite substratematerial. having a high bond strength can be formed. However, the waferbonding techniques have the problems in that wet processing must beperformed for cleaning the substrates and therefore production stepscannot be kept simple.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a process forproducing a composite substrate material, wherein wet processing neednot be performed, and a composite substrate material having a high bondstrength, high long-term reliability of a bonded area, and goodenvironmental temperature resistance characteristics is capable of beingproduced.

Another object of the present invention is to provide a compositesubstrate material having a high bond strength, high long-termreliability of a bonded area, and good environmental temperatureresistance characteristics of the bonded area.

The present invention provides a process for producing a compositesubstrate material, comprising the steps of:

i) overlaying a functional film, which has photo-catalytic effects, on asurface of each of at least two substrate materials,

ii) exposing the functional film, which has been overlaid on the surfaceof each of the at least two substrate materials, to light having awavelength falling within an absorption wavelength range of thefunctional film, and

iii) bonding the at least two substrate materials to each other with thefunctional films, which have been exposed to the light, interveningbetween the at least two substrate materials.

In the process for producing a composite substrate material inaccordance with the present invention, as the functional film, a filmcontaining TiO₂ should preferably be employed. In such cases, the lightirradiated to the functional film should preferably be ultravioletlight. Also, in cases where a film other than the functional film isalso overlaid on the surface of each of the at least two substratematerials, the functional film should preferably be formed as a toplayer.

The present invention also provides a composite substrate materialproduced with the process in accordance with the present invention.

Specifically, the present invention also provides a composite substratematerial, comprising at least two substrate materials bonded to eachother with functional films, which have photo-catalytic effects,intervening between the at least two substrate materials. In thecomposite substrate material in accordance with the present invention,each of the functional films should preferably be a film containingTiO₂.

Also, in the composite substrate material in accordance with the presentinvention, each of the functional films should preferably also act as ananti-reflection film. Further, each of the functional films may beformed thin such that each of the functional films is opticallynegligible.

With the process for producing a composite substrate material inaccordance with the present invention, when each of the functional filmshaving the photo-catalytic effects, such as metal oxide films, typicallyTiO₂, is exposed to the light having a wavelength falling within theabsorption wavelength range of the functional film, the photo-catalyticeffects of the functional film occur. As a result, organic substances,and the like, which cling to the surface of each of the substratematerials, are decomposed approximately perfectly, and asuper-hydrophilic state occurs on the surface of each of the substratematerials. Thereafter, the substrate materials are combined with eachother such that the surfaces of the substrate materials, which surfacesare provided with the functional films, stand facing each other. Also, aload and heat are applied to the combined substrate materials. In thismanner, the substrate materials are bonded to each other with a highbond strength. The bonded area has high long-term reliability and goodenvironmental temperature resistance characteristics.

Further, with the process for producing a composite substrate materialin accordance with the present invention, wherein wet processing neednot be performed, the effects described above can be obtained easily.

Particularly, with the process for producing a composite substratematerial in accordance with the present invention, wherein each of thefunctional films also acts as the anti-reflection film, a compositesubstrate material, which is substantially free from reflection loss atthe bonded area or exhibits little reflection loss at the bonded area,can be obtained. Also, with the process for producing a compositesubstrate material in accordance with the present invention, whereineach of the functional films is formed thin such that each of thefunctional films is optically negligible, a composite substratematerial, which is substantially free from scattering loss at the bondedarea or exhibits little scattering loss at the bonded area, can beobtained.

In this manner, for example, a reflectivity at the bonded area of aplurality of optical substrates (or crystals) can be limited easily andreliably to a value of approximately at most 0.2%. Also, it becomespossible to directly bond laser crystals, optical wavelength convertingcrystals, or other crystals in solid lasers, and the like, to eachother. Therefore, with the process for producing a composite substratematerial in accordance with the present invention, a subminiature solidlaser having reliable performance can be furnished.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D are schematic views showing steps in a firstembodiment of the process for producing a composite substrate materialin accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinbelow be described in further detailwith reference to the accompanying drawings.

FIGS. 1A, 1B, 1C, and 1D show steps in a first embodiment of the processfor producing a composite substrate material in accordance with thepresent invention.

Firstly, as illustrated in FIG. 1A, two Si wafers 10 and 20, which actas substrate materials, are prepared. The Si wafers 10 and 20respectively have surfaces 10 a and 20 a, which have been subjected tomirror finish. Thereafter, TiO₂ thin films 11 and 21, which act asfunctional films having photo-catalytic effects, are respectively formedon the surfaces 10 a and 20 a. In this embodiment, each of the Si wafers10 and 20 has a thickness of 300 μm. Also, the TiO₂ thin films 11 and 21can be formed with, for example, a sputtering technique or a sol-geltechnique.

Thereafter, as illustrated in FIG. 1B, the surface of each of the TiO₂thin films 11 and 21 is exposed to ultraviolet light 30 having awavelength of at most 380 nm. As a result, the photo-catalytic effectsof the TiO₂ thin films 11 and 21 occur. With the photo-catalyticeffects, organic substances, and the like, which cling to the surfaces10 a and 20 a of the Si wafers 10 and 20 respectively, are decomposedapproximately perfectly, and a superhydrophilic state occurs on each ofthe surfaces 10 a and 20 a.

Thereafter, as illustrated in FIG. 1C, the Si wafers 10 and 20 areimmediately combined with each other such that the surfaces 10 a and 20a stand facing each other. As illustrated in FIG. 1D, a load and heatare applied to the Si wafers 10 and 20, which have thus been combinedwith each other. In this step, the load is set at, for example,approximately 500 g/cm². Also, the heating is performed, for example, ata temperature of 300° C. for one hour.

Thereafter, the Si wafers 10 and 20 are cooled and the load is removedfrom them. In this manner, wafer bonding is finished. When the bondstrength of the Si wafers 10 and 20 was measured with a push-pull gauge,it was confirmed that a bond strength of at least 2 kg/cm² could beobtained.

A second embodiment of the process for producing a composite substratematerial in accordance with the present invention will be describedhereinbelow. In the second embodiment, a quartz glass plate and a LiNbO₃wafer are employed as the substrate materials. The quartz glass platehas a refractive index of 1.45 with respect to light having a wavelengthof 550 nm, has a thickness of 300 μm, and has been subjected to mirrorfinish. The LiNbO₃ wafer has a refractive index of 2.25 with respect tolight having a wavelength of 550 nm, has a thickness of 500 μm, and hasbeen subjected to mirror finish. Also, TiO₂ acting as a high refractiveindex material, SiO₂ acting as a low refractive index material, MgF, andthe like, are overlaid on each of one surface of the quartz glass plateand one surface of the LiNbO₃ wafer. In this manner, a multi-layer film,which acts as an anti-reflection film with respect to the refractiveindexes of the quartz glass plate and the LiNbO₃ wafer when the quartzglass plate and the LiNbO₃ wafer are combined with each other, isformed.

At this time, the film formation is designed such that the TiO₂ thinfilm constitutes the top layer on the surface of each of the quartzglass plate and the LiNbO₃ wafer. Thereafter, the surface of each of theTiO₂ thin films overlaid on the quartz glass plate and the LiNbO₃ waferis exposed to ultraviolet light having a wavelength of at most 380 nm.As a result, the photo-catalytic effects of the TiO₂ thin films occur.With the photo-catalytic effects, organic substances, and the like,which cling to the surfaces of the quartz glass plate and the LiNbO₃wafer, are decomposed approximately perfectly, and a super-hydrophilicstate occurs on each of the surfaces of the quartz glass plate and theLiNbO₃ wafer.

Thereafter, the quartz glass plate and the LiNbO₃ wafer are immediatelycombined with each other such that the surfaces, which have been set inthe super-hydrophilic state, stand facing each other. Also, a load andheat are applied to the quartz glass plate and the LiNbO₃wafer, whichhave thus been combined with each other. In this step, the load is setat, for example, approximately 500 g/cm². Also, the heating isperformed, for example, at a temperature of 300° C. for one hour.

Thereafter, the quartz glass plate and the LiNbO₃ wafer are cooled andthe load is removed from them. In this manner, wafer bonding isfinished. When the bond strength of the quartz glass plate and theLiNbO₃ wafer was measured with a push-pull gauge, it was confirmed thata bond strength of at least 2 kg/cm² could be obtained.

Also, when optical reflection loss at the bonded area between the quartzglass plate and the LiNbO₃ wafer was measured, it was confirmed that theoptical reflection loss was restricted to a value of at most 0.2%, andthat the multi-layer coating film containing the TiO₂ thin film acted asthe anti-reflection film.

A third embodiment of the process for producing a composite substratematerial in accordance with the present invention will be describedhereinbelow. In the third embodiment, two quartz substrates are employedas the substrate materials. Each of the two quartz substrates has arefractive index of 1.45 with respect to light having a wavelength of550 nm, has a thickness of 300 μm, and has been subjected to mirrorfinish. Also, a TiO₂ thin film is formed to a thickness of at most 50 nmon each of the surfaces of the quartz substrates. Thereafter, thesurface of each of the TiO₂ thin films overlaid on the quartz substratesis exposed to ultraviolet light having a wavelength of at most 380 nm.As a result, the photo-catalytic effects of the TiO₂ thin films occur.With the photo-catalytic effects, organic substances, and the like,which cling to the surfaces of the quartz substrates, are decomposedapproximately perfectly, and a super-hydrophilic state occurs on each ofthe surfaces of the quartz substrates.

Thereafter, the quartz substrates are immediately combined with eachother such that the surfaces, which have been set in thesuper-hydrophilic state, stand facing each other. Also, a load and heatare applied to the quartz substrates, which have thus been combined witheach other. In this step, the load is set at, for example, approximately500 g/cm². Also, the heating is performed, for example, at a temperatureof 300° C. for one hour.

Thereafter, the quartz substrates are cooled and the load is removedfrom them. In this manner, wafer bonding is finished. When the bondstrength of the quartz substrates was measured with a push-pull gauge,it was confirmed that a bond strength of at least 2 kg/cm² could beobtained.

Also, it was confirmed that no scattering loss occurred at the bondedarea between the quartz substrates. Ordinarily, in cases where the filmthickness of the functional film, such as the TiO₂ thin film, is set ata value of at most 50 nm, the presence of the functional film becomesoptically negligible.

In the embodiments described above, two substrate materials are bondedto each other. However, the process for producing a composite substratematerial in accordance with the present invention is also applicablewhen three or more substrate materials are bonded to one another.

In addition, all of the contents of Japanese Patent Application No.11(1999)-254350 are incorporated into this specification by reference.

What is claimed is:
 1. A process for producing a composite substratematerial, comprising the steps of: i) overlaying, on a surface of eachof at least two substrate materials, a bonding film which is capable ofgenerating photo-catalytic effects and causing a super-hydrophilic stateto occur on said surface, ii) exposing the bonding film, which has beenoverlaid on the surface of each of the at least two substrate materials,to light having a wavelength falling within an absorption wavelengthrange of the bonding film to generate said photo-catalytic effects andto cause said super-hydrophilic state to occur on the surface, and iii)bonding the at least two substrate materials to each other with thefunctional films, which have been exposed to the light, wherein thebonding films are positioned between the at least two substratematerials.
 2. A process as defined in claim 1 wherein the functionalfilm is a film containing TiO₂.
 3. A process as defined in claim 2wherein the light irradiated to the functional film is ultravioletlight.
 4. A process as defined in claim 1, 2, or 3 wherein a film inaddition to the bonding film is overlaid on the surface of each of theat least two substrate materials, and the functional film is formed as atop layer.